Optical distance sensors measure the distance of a given target from a reference spot such as the position of the measuring sensor. Various distance measurements sensors have been developed over the years that involve a variety of technologies including ultrasonics, the use of sound waves, microwave/millimeter wave radar using pulsed radio frequency energy, and more recently, laser radar using electrically modulated optical energy.
Distance sensors are vital for numerous applications such as industrial non-destructive testing, reverse engineering, virtual reality, machined parts quality controls, machinery operations, civil engineering, architecture, and design and testing of large to micro-sized structures.
A specific application of a distance measurement sensor is a liquid level measurement device. Optical liquid level sensors can be basically divided into two classes. Intrusive optical fiber-based sensors that have direct interaction with the liquid and remote laser radar type sensors based on pulsed lasers and temporal signal processing of sampled received data.
The optical method for distance measurement is a preferred method in clear line-of-sight measurement and testing cases with optically specular (or retroreflective) or diffused (optically scattering) targets as laser beams can have high power, low divergence, ultra-short pulse widths, and small far-field spots due to the small (e.g., micron) wavelength size. In effect, distance along z-direction measurements can be conducted over long distances with high spatial resolutions, along x and y coordinates of an x-y-z Cartesian coordinate system, leading to accurate three-dimensional profiles of a target surface or structures.
Today, temporal/frequency processing of the electrically/optically modulated laser beams is used to deduce the distance/profile of the target under observation. For example, a continuous wave laser beam is amplitude modulated by a short pulse waveform of a given pulse wide and a given Pulse Repetition Frequency (PRF). The PRF sets the maximum distance the sensor can measure while the pulse width sets the best distance measurement resolution. To measure over long distances, the PRF rate is lowered while for better resolution, the pulse width is shortened. Low PRFs means a slow sensor response time while a shorter pulse (e.g., going from microseconds to nanoseconds) implies broader instantaneous bandwidth electrical processing of the received signal making the electronic hardware considerably more expensive. With ultra-short (e.g., picoseconds) laser pulses, this situation on wideband processing is further exacerbated as special electronics is required for processing. In addition, ultrashort pulse optical radiation means the optical spectrum is broad and in effect, target response over the spectrum can be different. In short, today, relative distance measurements over wide ranges and high resolution requires expensive and spectrum sensitive temporal electrical processing. Although some laser beam processing methods including speckle interferometry have been used for relative distance measurements such as in surface profiling of target objects like large mirror optics or turbine engine blades, unlike laser radar, these methods have a restricted dynamic range for relative distance measurement. Fundamentally, all optically or electrically interferometric methods are very sensitive to external (optical path or target motion) or laser phase noise, limiting the operational robustness of such distance sensors. More recently, self-modulation interferometry of laser beams has also been tested showing improved robustness to interferometric noise. Nevertheless, all these laser time-frequency modulation methods of laser-based distance sensing add cost to the overall sensor system.
What is needed to solve the problems association with the previously described prior art is a relative distance sensor that has high distance dynamic range, excellent distance measurement resolution, high spatial profiling resolution, and low cost. The present invention is a remote optical distance sensor that does away with the limitations associated with the classic laser radar temporal/frequency processing approach. Specifically, a novel remote distance measurement hybrid optical sensor is described using smart agile laser beam optics and robust spatial optical processing leading to the desired powerful distance sensor.